[NLRS] 2m Pager? noise

Sat Oct 28 23:03:18 EDT 2017

Sorry-- I missed the part about the 746PRO not having an issue while the SDR dongles do so I see that it is not the front end getting slammed-- assuming all of those always follow the ARR preamp.

Chris N0JCF

On October 28, 2017 9:45:09 PM CDT, Chris Elmquist <chrise at pobox.com> wrote:
>
>
>Yes. True.  Well, maybe the preamp is getting crunched then and a newer
>design with high IP3 would be a solution?
>
>Tricky.
>
>Chris N0JCF
>
>On October 28, 2017 8:45:06 PM CDT, Mary Brown <maryalanab at gmail.com>
>wrote:
>>Problem is the frequency is so close that any filter is going to drop
>>wanted
>>signals badly if it is going to be effective enough to work in so
>>close...
>>you could design for a single center frequency then use an
>interdigital
>>filter(big, bulky on 2m, hard to construct...) and not stray more than
>>20kc
>>or so either side...
>>
>>Mary
>>W0AAT
>>
>>-----Original Message-----
>>From: nlrs-bounces at mailman.qth.net
>>[mailto:nlrs-bounces at mailman.qth.net] On
>>Behalf Of Chris Elmquist
>>Sent: Saturday, October 28, 2017 8:09 PM
>>To: Kirk P
>>Cc: nlrs at mailman.qth.net
>>Subject: Re: [NLRS] 2m Pager? noise
>>
>>
>>
>>You might consider a 1/4-wave open-end or 1/2-wave shorted stub notch,
>>tuned
>>for 144.390.  The idea would be to try to knock down the levels you
>see
>>on
>>144.390 so that they are no longer "slamming" you.
>>
>>This is basically a short piece of coax, carefully tuned in length and
>>put
>>in parallel with your Rx path with a T-connector.
>>
>>They can be tricky to get on frequency and the Q (sharpness of the
>>notch) is a function of the choice of coax.  A piece of RG58 will be
>>very
>>broad and not too good of a notch while building it with a piece of
>>7/8"
>>heliax might do pretty good.
>>
>>I found some information here,
>>
>>http://www.radagast.org/~dplatt/hamradio/K6OIK-filters.pdf
>>
>>There is also a discussion in some of the older (cc. 1980's) ARRL
>>handbooks
>>I believe.
>>
>>The length is a function of the velocity factor of the coax used and
>>includes all the connectors and the T, etc in the calculation.
>>Some people start with a longer length coax "tail" and keep trimming
>>and
>>testing until they have achieved the desired notch.  This is more
>>difficult
>>if it's a piece of 7/8" heliax :-)
>>
>>If you put this ahead of your first stage pre-amp, that might have a
>>negative affect on your noise figure-- although, hearing nothing due
>to
>>APRS
>>packets also has a negative affect on your noise figure ;-)
>>
>>You might be able to put it after the preamp but then that's where the
>>high
>>IP3 LNA mentioned may be important so that it isn't being crunched
>>first.
>>Dr. Gerry can correct me here but basically high IP3 means the preamp
>>can
>>tolerate much higher RF levels at the input before it saturates and
>>starts
>>to distort the output signal.  A distorted output signal will contain
>>mixing
>>products that appear as intermod or ghost signals in your receiver.
>>
>>PAR's "no way" response was to the question of building a 144.390
>notch
>>or
>>??
>>
>>Chris NØJCF
>>
>>On Sunday (10/29/2017 at 12:21AM +0000), Kirk P wrote:
>>>
>>>
>>> I sure appreciate all the information and ideas. Yes, Im sure its
>>APRA as
>>thats its freq.
>>>
>>>
>>> Not sure there is anything now that I can do as they are operating
>>within
>>their rights though some seem awfully over modualted.
>>>
>>>
>>> It would appear that my only hope would be to find some type of
>>filter
>>> I can set for a freq up higher in band that can just catch .380 on
>>its
>>> lower edge..a cavity that I can adjust perhaps? I dont know..so much
>>> of this VHF stuff is newer to and above my knowledge..HF Id have it
>>> solved already HI HI
>>>
>>>
>>> I modified my 746PRO to accept a SDR RX as its RX...I use both the
>>Funcube
>>PRO+ as well as RTL dongles for 160m-2m utilizing the 746 as the TX
>>radio.
>>Amazed at how well the RX is now that Ive done this..but it appears to
>>come
>>at a price..the dongles have a bandwidth of up to 1mhz of RX and the
>FC
>>at
>>196khz and fold under such strong local signals when coupled with
>WSJTX
>>for
>>EME.
>>>
>>>
>>> If I use just th746PRO as the RX in its original configuration (Yes,
>>I can
>>switch back and forth the way I did the mod) it handles this noise up
>>the
>>band much better..though doesnt hear weak signal as well as the SDR
>>using
>>SDR console software.
>>>
>>>
>>> Anyone that give me filtering ideas Im open to suggestions.
>>>
>>>
>>> PAR electronics got back to me and said no way.
>>>
>>>
>>> They did mention a HIGH IP3 LNA as an option though Im not familiar
>>with
>>those. I currently use the ARR 144 LNA.
>>>
>>>
>>> Kirk, N0KK
>>
>>--
>>Chris Elmquist
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>
>-- 
>Chris Elmquist
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-- 
Chris Elmquist